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%I #22 May 20 2017 21:41:42
%S 1,2,6,24,120,685,4148,25661,159829,997870
%N Number of permutations of length n sortable by a pop stack followed by a queue and then by a stack.
%H R. Smith and V. Vatter, <a href="https://ajc.maths.uq.edu.au/pdf/58/ajc_v58_p157.pdf">A stack and a pop stack in series</a>, The Australasian Journal of Combinatorics, Volume 58(1) (2014), pp. 157-171.
%e There are only a(6)=685 permutations of length 6 that can be sorted by this machine; the machine can sort all shorter permutations.
%Y Cf. A033321.
%K nonn,hard,more
%O 1,2
%A _Vincent Vatter_, Mar 06 2013
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